This invention relates to a print head for optical printing devices.
High speed printing devices used in data processing systems are intended for translating electric input signals into a visual representation readily recognizable in a printed form.
Of such printing devices, this invention is applicable to an optical printing device using light sources and photosensitive means, more particularly to an optical printing device using solid state light emitting devices.
For the light emitting devices, linear arrays of light emitting diodes or semiconductor lasers are employed.
An array of light emitting devices may, for example, consist of gallium arsenide phosphide as a base material having N layers of Ga As P formed by means of an epitaxial growth process, and a large number of P layers in a linear configuration formed by diffusion of Zn. However, the maximum available size of gallium assenide phosphide as the base material is limited approximately to five centimeters in terms of its wafer diameter.
For the purpose of extending the printing width of a printer, a plurality of light emitting device arrays are employed and it is necessary to align light images from said light emitting devices in a single straight image line on the photosensitive surface. Hitherto, for an optical printing device, light emitted from each individual light emitting device in an array is coupled to the photosensitive surface by means of each individual optical fiber cable arranged opposite to the corresponding light emitting device.
Said light emitting device and one end of said opposite optical fiber cable either come into contact with each other or are closely disposed leaving a minor spacing which may not exceed the order of several microns. The other end of the optical fiber cable and the photosensitive surface are similarly disposed leaving a spacing which may amount approximately to one hundred microns.
In this case, said respective spacings are extremely delicate as exemplified and therefore must precisely be maintained since the space intervals in excess of the foregoing values will adversely affect the light transmission properties and the extent of light spots, namely the printing quality. It is, however, extremely difficult to keep precisely said space intervals. Moreover if a fiber cable collides with the photosensitive surface, with the spacing between the fiber cable and the photosensitive surface on the drum failing to be provided correctly, the fiber cable and the photosensitive surface may be damaged due to the rotation movement of the drum.
In the case of a fiber cable, its image plane is located at the tip cross-section thereof, whereby the focal depth can be available only inside the cable.